This invention relates to certain amorphous fluoropolymers which are particularly suitable as cladding materials in optical fiber constructions as well as in certain electronics applications, molded articles, and films.
While tetrafluoroethylene homopolymers and copolymers have many excellent properties, they usually suffer from low modulus, especially at elevated temperature; poor creep resistance; insolubility; and in some cases intractability. Various fluoropolymers have been proposed from time to time for cladding optical fibers, mainly because of their low refractive indices. A good polymeric cladding material for optical fibers should be completely amorphous because crystallites present in polymers would cause light scattering. Further, it should have a high glass transition temperature, T.sub.g, especially if intended for use at high temperatures because above its T.sub.g it would lose some of its desirable physical properties and in particular it would be unable to maintain good bonding to the fiber core. A desirable T.sub.g would be above 140.degree. C., preferably above 180.degree. C., especially above 220.degree. C. Suitable, entirely amorphous fluoropolymers having significantly high T.sub.g 's have not been heretofore reported.
U.S. Pat. No. 3,978,030 to Resnick describes certain polymers of perfluoro-2,2-dimethyl-1,3-dioxole (PDD), which has the following formula: ##STR1## The above patent describes both homopolymers of PDD, which are not further characterized, and a crystalline copolymer with tetrafluoroethylene (TFE), which has a melting point of about 265.degree. C.
Since Resnick's discovery of PDD homopolymer, it has been established that the material is amorphous and has a very high T.sub.g of about 330.degree. C. The homopolymer, however, is not readily melt-processible because of poor flow and some degradation.
Crystalline copolymers of PDD with TFE cannot be used in many applications, where, for example, optical clarity, dimensional stability, solubility, or high T.sub.g is required.
The polymers of U.S. Pat. No. 3,978,030 have thus not been produced commercially, in spite of the fact that perfluoro polymers having desirable properties would have many possible uses in various high technology applications.
It has now been found that the dioxole PDD forms amorphous copolymers that have unique properties that make them particularly suitable for a number of special applications requiring high performance.